Bombardment induced conductivity in solid insulators



A. J. AHEARN 2,604,596

CED CONDUCTIVITY IN SOLID INSULATORS July 22, 1952 BOMBARDMENT INDL' Filed Aug. 14, 1948 SCOPE L FIER IL IL4MP I FIG. 4

AMPLIFIER FIG. 5

|- MOD.

A TIORNEY Patented July 22, 1952 BOMBARDMENT 2,604,596 INDUCED CONDUCTIVITY 1 IN SOLID lNS-ULATORS Arthur J- Ahearn, Mountainside, N. J., assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application August 14, 1948, Serial No. 44,377

This invention relates to bombardment induced conductivity in solid insulators and to applications thereof in the electrical arts, and is an improvement over the invention of D. E. Wooldridge, Serial No. 747,888, filed May 14, 1947, now Patent 2,537,388 issued January 9,1951. The improvement consists in the irradiation by light of the'solid insulator in question while the insulator is being bombarded inaccordance with the Wooldridge principle; and results in increased current and the reduction of polarization effects. To the extent that the invention aims at substantially the same result as by the use of a certain McKay improvement of the Wooldridge invention, itmay be thoughtof as an-alternative or additive to that invention as; disclosed in K. G. McKay, Serial No. 789,667 file'd December 4, 1947, now Patent 2,543,039 issued February 27, 1951, relating'to the use of an alternating voltage, instead of a direct voltage; impressed on-the solid insulator. V "if Applicant has discovered that'in a diamond in which current pulses are induced byfalpha'particlebombardment while there is an electrical potential across the diamond-more pulsesare large enough to be observed when light is directed into the diamond than when it is in the dark. While the effect which this light hasupon the process may not be entirely understoodapplicant believes it to be the following at least in part. When a current pulse is induced in the diamond, the carriers of charge, electrons and"holes, proceed toward the respective external electrodes at a certain speed. Some of-these become trapped at imperfections in the diamond lattice structure and are lost from the stream of current. At the same time these trapped carriers cause a polarization which impedes the passage of 'other similar carriers, and makes their trapping more probable. reduced to a very low value before all the carriers cancross the diamond-2 Whennow light is.

- The eff cacy -of applicant's light irradiation principle to reduce'or avoid polarization effects is therefore reflected in an-increased-current response underg'iven conditions; of field strength and energy of bombardment. The invention may be thoughtof as providing with quite different The. result is that the current is rapidly '11 Claims (Cl. 250-833) means an end result similar to that of McKay.

The McKay invention provides a procedure whereby the direction of flow of current is reversed so often that strong polarization can not build up while in applicants invention polarizationtends' to be destroyed-as rapidly as it is built up. While applicant has achieved success with his method by using alpha particle bombardment ofdiamond, he has every reason to think that like efiectswouldattend bombardment by other types of charged particles; such protons, electrons and beta particles. In fact, in this, as in other respects, the invention is 'quite comparable to the above Wooldridge-McKay alternatives so that the choice of incident radiation or bombardment may range as widely as to include X-rays, gamma rays, and other short-wave electromagnetic radiations. Because of this versatility the term radiation will be used in this specification to include these various types of bombardment, by particles or "by waves. "For like reasons the choice of solid insulator is as'wide as the instances of said Wooldridge and McKay inventions and as enunciated in their patents as above; although applicants workhas been largely with diamond, which by its special characteristics, relatively to other crystal forms, as pointed in said applications, typically and well illustrates the operation of the bombardment induced conductivity principle. Likewise, the field of practical utility of applicants invention is the same as, or comparable with, those of said Wooldridge and McKay inventions, some of which will be given special consideration in the detailed description that follows.

Preliminarily to an explanation of the polarization phenomena which brought this invention into being and which suggest the obje'cts and features of the invention; it is in order to orient. the subject of the invention and provide a necessary minimum of background for. it. 7 For further elaboration, the Wooldridge and McKay specifications may be referred to. t V

The phenomenon of bombardment induced conductivity in solid insulators is an instance of valve action. Correspondingly as the vacuum tube is made conducting under the influence of electrical means (control grid potential) independently "of "the voltage; applied between the other instances of the flow of electricity in conducting media. Each unit of this radiation (note that in the usual practical case it would be a bombardment of charged particles), depending on its energy, can remove a number of valence electrons from their bonds in the material of said solid insulator in such a way that, as multipled by the number of units of radiation, and by the chain reaction consequent on'th'e initial action, render the solid insulator temporarily conducting.

The bombarding particles (here, and hereafter in this specification, for simplicity and convenience only, the action of the invention will be visualized in terms of bombarding, charged particles) penetrate the insulator, causing a disruptive separation of the positive and negative charges specific to the atoms which are affected thereby. These charges are drawn towards the electrodes which induce the field in which the insulator is contained, by the potential therebetween. This motion of charges constitutes a conduction current, which may be suitable amplified and measured by conventional apparatus.

When in this Way a charged particle bombardment removes a valence electron from its bonds in an insulating target, producing a deficiency of one electron in the atomic structure imme diately affected, this localized electron deficiency is. called; a hole.. field the arrangement of electrons is. changed, and the location f any given hole is similarly changed. As a consequence, the hole may be conveniently regarded as a positive particle which is free to move under the influence of the field; similarly the electron freed from the bond in question constitutes a negative particle likewise free to move under the influence of the field.

If there is no applied field, any free electron or a positive hole moves in virtue of thermal agitation and consequently has a completely random motion. Under an applied electric field, there is a'directional motion superposed upon the random one. The order of mobility of the 'electrons in diamond is about 150 centimeters per second for a field of one volt per centimeter. (F. Seitz, Physical Review 73, 549, 1948.) The mobility of the electrons is affected by the number of traps, that is, by the presence of foreign atoms or imperfections in the crystal. If an electron gets into a trap, it takes a greater or less amount of time to get out, depending upon the thermal energy required. If the time which a free electron spends moving in. the crystal before being trapped is, on the average, less than the transit time, many of the electrons freed by the bombarding particle will effectively move only part of the distance through the crystaland thus will not actually be collected on the electrode. Although this movement of charge through part of thejcrystal will contribute to the total observable conduction current, the contribution will'be less than if the electron had been collected on an electrode. Similar considerations concerning mobility and trapping also apply to conduction by positive holes. In order to minimize the number of effective traps in a given target, so as to realize a substantial conductive current, the length of path in the target, between the electrodes, should be made as small as possible. Also, because of the effect of the fields of the trapped charges in the target, on the corresponding free electrons. which tends to counter balance the pull of the superposed field from the electrodes, said field should exceed a threshold value de- Under an applied electric pendent on the precise conditions; that is, the potentials between said electrodes should exceed a certain threshold value.

Consistently with the above and with further considerations to be recited later in this statement of invention, the primary object of this invention is to increase the bombardment induced conductivity response in solid insulators over that obtained by the prototype Wooldridge invention.

A subsidiary object of the invention, suggested by the principal factor which tends to limit the response in the prototype invention, is to eliminate or avoid the phenomenon of polarization or the like in bombardment induced conductivity applications.

As in accordance with the invention, the polarization isv eliminated or avoided by irradiation of the solid insulator in question while subjected to the radiation or bombardment Whichtends to induce electric conductivity therein in the presence of an electric field. Applicant has bombardedv diamond crystals with alpha. particles emanating from polonium which is also called radium F, with eminently satisfactory results. Although a considerable degreev of selection was necessary the test results were completely stable, repeatable and unambiguous. As has been expressed before the invention. is useful in every application of the bombardment induced conductivity principle which has so farbeen suggested, and particularly as suggested in theabove Wooldridge and McKay specifications. Thefollowing is adduced not only to indicate applicant's reasons for relating his results'to the phenomenon of polarization but to teach the public how to operate hisinvention by a resum V of his own essential procedure.

When a suitable diamond (for example) with appropriate electrodes across which a suiiiciently high voltage was applied, was bombarded with alpha particles from polonium, conductivity pulses were observed each of which was specific to an individual alpha particle. When the direction of the applied field was reversed the direction of the pulses was also reversed. When the voltage was reduced to zero, the pulses were observed for a short time, in general opposite in direction to thosev that occurred when the voltage was applied. Quite clearly these pulses were space charge pulses dueto a polarization of the diamond medium, set upby the trapping of electrons or positive holes (depending on the direction of the applied field) before they have traveled completely through the diamond to the electrode. In general, in the absence of light, with a given rate of bombardment by radiation of a given quality (e. g. alpha particles from polonium) the pulse rate at first rises rapidly with increasing applied voltage and then approaches a limiting valueiue. the pulse rate r approaches voltage saturation. The term voltage saturation, as here used, is obviously aha-- logous to the term as used in electronics to describe the situation Where there is more than enough voltage to attract to the anode all of the electrons available at the surface of" the cathode.

As is experimentally observed, the build up of a given amount of polarization at voltages well below the voltage saturation region should decrease the pulse rate by an amount governed by the decrease in the resultant electric field in the diamond. However the same amount of polarization decreases the pulse rate in the neighborhood of voltage saturation by a relatively smaller amount. ,Similarly the effect of the light in increasing the pulse rate is greater at low voltages than is the case at voltages which approach saturation. 1

This phenomenon of polarization in diamond and the like solid insulators appropriate to bombardment induced conductivity is a serious limiting factor in the practice of the bombardment induced conductivity principle since, in ordinary cases, the choice of voltage to insure voltage saturation at all times would not be consistent with the optimum voltage as determined by other considerations. This matter of a proper choice of voltage (that is, proper choice of field intensity) and the related matter of the proper choice of insulator thickness, is treated in the above McKay application. 7

The eificacy of the use of irradiation of the crystal insulator, as in accordance with the invention, is evident from the following resum of applicants experiments, which will be better appreciated and understood after a reading of the illustrative disclosures of apparatus that was used or might have been used.

A diamond (for example) was mounted in an evacuated bell jar with a potential of about 200 volts applied between very thin electrodes on opposite major faces and one entire face was subjected to intense alpha particle bombardment from polonium. It is known that this bombardment could be incident on one of the electrode faces, or any other face of the diamond, and not necessarily in such a way that the motion of the incident radiation is in the direction of the voltage field. Under these conditions the alpha bombardment induced conductivity response (the number of pulses and the height of the pulses) decreased markedly in the course of a minute or so from the initial value, when the voltage was first applied. The test was repeated while the diamond was illuminated with light from an ordinary tungsten lamp. The response, as above measured, was markedly greater than for the previous test after the onset of polarization. The high response persisted as long as the diamond was illuminated but dropped back to the low value determined by polarization in the course of several seconds after the light was turned off. The response was comparable with that initially observed when the volttage was applied, without the illumination, at which time polarization had not had time to become effective, this indicating that the resultof the illumination was an effective antipolarization means, although of course there might be other phenomena coexistent with this.

The theory of polarization in this art is taught in greater detail than would be justified in the present specification, in the above Wooldridge and McKay specifications. It has been treated very briefly, and in general, above. The theory as related to the irradiation of the solid insulator by a light may, it is submitted, be accu rately stated as follows.

With an applied voltage the alpha .(for example) bombardment induced conductivity response decays with'time because the polarization which is built up by the trapped charges (electrons or positive holes) reduces the applied field. The illumination increases the response during the period that would otherwise be characterized bydecay by eliminating or diminishing this polarization. The actioneifects the release of the electrons or positive holes from their traps.

- =6 According to the modernband theory of sol-. ids, an electrical insulator like diamond can become conducting, a s in bombardment induced conductivity, only when electrons are given at least enough energy to jump across a region of forbidden energy valuesin the energy spectrum. According to the literature, this forbidden region in diamond is about seven electron volts wide. The alpha particle and electron bombardment induced conductivity experiments, including those employing illumination and alternating potentials across the electrode are consistent with this theory because they indicate that the electrons in these conductivity pulses have received ten or more electron volts of energy.

Inthe neighborhood of crystal imperfections,

however, the theory v predicts allowed energy levels in this normally forbidden region. These levels constitute electron traps. When electrons in conductivity pulses fall into these traps, they set up a space charge, that is, the diamond or the like becomes polarized. Depending on the nature and degree of the crystal imperfection, these electron traps may be shallow or relatively deep. That is, the electron trap energy levels may be just below the top of the forbidden energy region or they may be relatively far below the top.

In the illumination experiments all wavelengths shorter than 3000 A. U. were excluded by glass. In other words, the energy of the light quanta which increased the bombardment induced conductivity response was not greater than about four electron volts. These light quanta would, therefore, have enough energy to remove electrons from traps which were at most four electron volts deep. It is believed that light of shorter wavelength might be effective in increasing the bombardment induced conductivity response with respect to electrons which are trapped in deeper traps. Reference is made for background in this general field to Color Centers in Alkali Halide Crystals by F. Seitz, Reviews of Modern Physics, volume 18, Number 3, pages 384-408, July 1946.

Other objects andteachings of the invention are derivable from the detailed description hereinafter following, with reference to the accompanying drawings in which:

Figs. 1 and 2 illustrate'two preferred methods of applying the necessary electric fields to the surfaces or parts of surfaces of the insulators in question, with relation to the incidence of the bombarding particles or radiation, while also being irradiated with light, Fig. 1 also showing diagrammatically a somewhat elaborated form of lighting system that has been used in practice, and Fig. 2 showing what might be the same lighting system in an even more diagrammatic form;

V Fig. 3 illustrates a system of the invention for indicating the presence of conductivity in an insulator which is affectedby the bombardment of charged particles while, as per the invention, irradiated by light;

Fig. 4 illustrates a system similar to that of Fig. 3 for indicating the presence of bombardment induced conductivity in an insulator, here specifically an apparatus for actually counting the incident charged particles, not restricted to a particular type of source of bombarding particles; and

Fig. 5 illustrates the application of the bombardment induced conductivity principle of the invention to a complete amplifier organization.

It should be understood, in what follows, that,

with respect to applicant-s invention, similarly as its prototypes; the incidentradiations may almost impartially be made up of" various common types of charged particlesalthough applicants' work has been mainly with alpha particles; which are positively charged" particles. Other types of charged 'particleshere applicable include ordinary electrons as emanate from the cathodes of the usual electronic devices, beta particles; which are essentially high speed electrons, and other kinds of radiation which may be analyzed in terms ofthe-charged particles' on which their constitution may Ice-postulated. Similarly as" a charged particle of a conventional type, as alhpagbet'a or electron particles, of sufficient energy canremovea valenceelectron from'itsb'onds' as taught herein and in the-McKay and Wooldri'dge patents, so also units- (photons) of electromagnetic radiation, as in X-rays and gamma rays (that is, generally electromagnetic radiation ex-- elusive ofI-l'ertzian; infra-red, visible or ultra-'- violet light, all ofwhich are l'ow'energy electromagnetic waves may possess'suiilci'ent energyto' cause the" removal of valence-electrons from their bonds in-such a way'that thesolid' insulator'is rendered temporarily conducting. Alpha and beta: particles usually; and a's'contemplated by the present disclosure, emanate from radio'- active material. It should also be understood that circuits or systems for evidencing the. fact of bombardment induced: conductivity; as in accordance' with the invention, donot diiferin' concept dependingon whether'the" ultimate result is a graphical showing ofthe conductivity, as on an oscillograph screen; ora response ina' device for obtaininga quantitative measure of the incidenceof the beam of radiation. This is true. although certainfigures of' the drawings are so differentiated in order to'indicate such choice of means in theinterest of special considerations. As has already been; intimated, thereis likewise a wide choice of" solidi insulator; as taught 'in the prototype" specification; although applicants work has largelyconcerned diamond;

Theforegoing generalization also appliesto. the particular electrode systems'and Figs. 1' andl2 illustrate two kihdsof" electrode systems that. may be almost impartially; used in any. of the systems described or' to. be described in this specification, although. a" particular, choicemay Thesettwo. a

be urgedbypracti'ca'l considerations. systems differ in: the nature of the coupling-of the electrodes to the solid dielectric substancenon which they," are superposed; I In Fig; L the two. electrodes'are mounted in a sid'e -by-sid -presen-.- tation" on the same surface of the diamondiinf question and, therefore; so: that, the conduction" current flows chiefly near 'its bombarded surface;

whereas in Fig: 2the"electrodes"'are mounted on.

ternative among the many'solid dielectrics. that.

are available. For reasons of expediencyonly it" will be -assumed; unless otherwiseistated, that; diamond is used-inthe -'organizati'ons; of the other figures.

Similarly as inthe McKayi'invention identified.

in the statement of the invention. herein; and similarly as in the Wooldridge inventionialso there identified, the solid insulator material may bechosen onthe following basis: Itishould have a high insulating characteristidsoi as tobe mostl amenable; without-ambiguity; to. the conditions The: word; diamond imposedby the type of phenomena being treated. To this end, 'and'forotherreasons as well' which are not fullyknownat thistime; the-insulator should have preferably not only good insulating qualities but also should be preferably of'a-single crystal type-wi-th a-high degree of 'chemical purity and'freedomfrom-inelastic strain or'other crystal defects. These considerations commend the use of diamond, quartz, zinc'sulfide, thealkali halides (including potassium chloride" and potassium bromide), magnesium oxide; calcium fluoride, sodium nitrate, topaz; sliver chloride. orthoclase, beryl; calcite, apatite, selenite, tourmaline, emeralds; extremely pure" silicon carbide, and stibnitei Several of these substances, notably diamond, zinc,v sulphide; magnesium= oxide, sili'-'' con carbide; and stibnite'; have been'used' in the basic studies of bombardment induced conductivity" and thereisevery good reason to-think that theffeatureof using an alternating voltage field, attributable to applicant, is" applicable to each of them.

Referring to Fig. 1'morespecifically; two conducting metal film electrodes" I and 2 are mounted on one surface of insulator 3'. The gap 4 separating the electrodes is relatively small and various widths from .001 to .008 inch have been successfully used'in bombardment induced con-- ductivitytests. These electrodes may be preparedbydividingthe diamond surface roughly in half-'bystretching a wire of appropriate diameter across-and in close contact with the surface and'then-evaporating a conducting metal layer, in vacuum, onto said surface. This layer can be made so thin as to be semitransparent, provided that its electricalresi'stance is so low as notto affect its electrical performance unfavorably. The shadow cast b'ythe wire provides a gap when the wireis'removedl This'gap would'have constant width and represented a uniformly high resistance thereacross'at' any point.

Thecharged'particles' (of course comprehending radiation generally as pointed out in the statement'ofinvention) are assumed to conform to a ray'or beamindicated generally'by reference numberiwhich-beamis incident on the diamond surface. of course the beam'tendsto be most effectivewhere itis'incident on thediamond surface atthe'gap'but; depending on the type of charged particles;.theele'ctrodes would no'tnecessarily impose a: substantial barrier however, the electrode systemof Fig; 1 requires that the bombarding'particles strike the gap or very closely adjacent thereto. Laternumbered figures will show,. more specifically and in detail, organizations including the elements which are here showntoia. large extent diagrammatically. The angle ofiincidencehas notbeen observed to be critical.

. A moderate direct voltage. applied betweenthese-electrodesbysource silproduced a relatively highelectri'c fieldinthe top surface layers of the diamond nearithe gap andtl'ie resultant induced conductivity pulses observed in the indicating I means, which i diagrammatically indicated as a meter;1pass' only acrossthese surfacelayers. In thestatement of inventionabove, certain quantitativei'valu'e's, ortheir criteria, have been indicated; this'applying; not only to this figure but to the-otherfigures asiwell.

On theorgamzation as above, which is the same as the correspondingly identified organization of the prototype Wooldridge application, there is superposed the'suborganization of the invention, here constituted by a source I 80f polyplacement.

chromatic light, the condensing lens I!) which is used to parallelize the beam for passage through the prism 20, which may be rotatable as shown so as to play the resultant beam over the aperture plate 2|. 'The eventual beam 22 emerging from the aperture is incident on the diamond. Of course the exposure should include that portion of the diamond through which the conduction pulses pass. In Fig.1 strictly as shown this would assume that some of the light penetrates the diamond to the vicinity of the slot 4. Should it be desired to increase the incidence of the light on that portion it would be a simple matter to angle the diamond so as to provide direct exposure of the electrode surface to the light. Like considerations apply to Fig. 2 to be described were elements [9, 20, and 2| of said Fig. l to be used with the diamond of this Fig. 2, as they might well be. The source I8 is described, and diagrammatically shown, as a polychromatic source only because of the related showing of lens, prism, and aperture plates, the whole suggesting a means for choosing light of a given wavelength by this combination of means, since the aperture will pass only a selected portion of the incident beam,selected according to a desired portion of the frequency spectrum of the light beam, as determined by the angular position of the prism 20. It should be understood, that although there may be a differentiation as to the depth of the traps afiectedby the light and depending onthe particularfrequency of the light waves involved, the use of light generally without discrimination'as to frequency would, in

the ordinary case, probably be reasonably effective in preventingthe polarization which inhibits the bombardment induced conductivity.

If the light is used without suchdiscrimination it would not have'to be condensed, neither would the beam have to have any particular configuration or afiect a limited portion of the diamond surface. In fact, applicant has found his invention effective whenthe light issuffused over-the whole diamond organization. It should also be understood that although, in the subsequently numbered figures, the expedient of the invention is represented merelyby a light beam and an aperture plate, the elaborate organization of Fig. 1 as to the invention should be equally well understood to be applicable thereto. In fact the aperture plate might be dispensed with so as to indicate the nature of the invention merely by a teaching of a beam of light incident on the diamond.

Fig. 2 presents the second type ofelectrode Here the electrodes I and 2 are placed on opposite sides of the diamond 3. A typical diamond specimen for this purpose might be about one-quarter inchin either principal dimension and-about .020 inch thick-.- Thus a potential difference of --1 00 volts fromdirect voltage source 6, across these electrodes, will produce a uniform electric fieldof about 2000 volts per centimeter throughout the body of the diamond. In this type of electrode placement some of the induced conductivity pulses, observed in the meter or other indicating device shown, pass completely through the body of the diamond as distinguished from the Fig. l-placement in which the pulses pass only in the region of the front surface. Similarly as in Fig. 1, the polarization within the diamond which, as explained in the statement of 'invention,,tends to accompany the achievement of bombardment induced conductivity where a ,directvoltage field is used,

. such as'gamma or X-rays.

is eliminated or prevented by irradiation of the diamond bylight represented by the beam 22 passing through the aperture in the aperture plate 2! and incident on the diamond in such a way as to provide a substantial penetration thereof. V

In Fig. 3, illustratingfa practical embodiment of a system operating according to the principles enunciated with respect to Figs. 1 and 2, like elements are again designated by like reference numerals. The diamond 3 is coated with metallic electrodes l and 2 as in Fig. 2. The whole is mounted in an evacuated receptacle 1. The charged particle source 8, first assumed as the source of alpha particles, may consist of a silver sheet 9 on which is deposited a layer of radium sulphate having a given density of radium atoms (in a typical instance, 12 micrograms of radium per square inch). Of course other sources of alpha particle emanation are well known in the art and may impartially be used in the Fig. 3 organization. In fact that organization may Well be used to explore the possibilities as to new sources of said emanations. The reference numeral l0 indicates diagrammatically a support for the silver sheet. In the prior art there are adequate teachings of mountings similar to this and the other elements here disclosed in an evacuated container. Other facilities, likewise taught by the prior art, could be used to advantage, such as a magnetic control means to determine the particular direction of incidence of the particles on the diamond, or even to adjust the position of the alpha particle, source in apposition to the aperture II in diaphragm-like element [2, for-fur ther determining and limiting the precise coaction of the beam of charged particles and the diamond. The particular means of the invention for irradiating the diamond with alight beam is disclosed similarly asinFig. 2'. 1

The same illustration is applicableto the use of a beta particle source and in'this instance the element 9 could have the form of a piece of glass on which a minute quantity of artificially radioactive strontium had been deposited. The same teaching extends, of course, to other sources of charged particles or electromagnetic radiation To suit the teachings of this Fig. 3, which discloses, outside of the light irradiation means of the invention, a more elaborate and complete organization than that of Figs. 1 and 2, the potentiometer l3 may be used as shown to determine a desired fractional part of the Voltage of the primary source 6, the voltage impressed therefrom being indicated by the voltmeter V. Of course in the specific instance of Fig.3, the bombarding particles penetrate the exposed upper electrode before affecting the diamond, this of course not representing a significant departure from the alternative in which the diamond is directly bombarded, providing this electrode be suificiently thin. The detecting circuit may comprise amplifier M and cathode-ray oscilloscope or the like I 5, both showing diagrammatically, to suggest the comparatively impartial choice of specific means to achieve these functions;

It is not a rigid requirement that the container be evacuated. In fact a rough vacuum is produced merely to eliminate-small induced conductivity pulses caused by ionization of the air produced by the charged particles in their transit 1.! particle .source .as .close aspracticable to the diamond, this therefore requiring that the diamond =3, source 8 and diaphragm l2 .all be very closely interspaced.

Fig. 4 emphasizes the embodiment of the invention as a counter, alternatively, for example, to the well-known Geiger counter. Similar elements are similarly designated as in Fig. 3, the essential difference being the using of the counting device [6 in place of the oscilloscope l5 of Fig. 3, it being recognized that the prior art provides impulse .(pulse) counters of a large variety and scope and'having a greater facility for count.- ing purposes than the oscilloscope of Fig. 3, although, as has been explained, it may be used quantitatively both to measure the intensity of any given pulse and the number of such pulses. A potential of the order of 200 or 300 volts may be applied across the diamond crystal. Of course a solid insulator other than a diamond crystal, within the spirit of the invention, may be used. The diamond crystal and electrode structure may be made to occupy a space each of whose dimensions is less than onevquarter of an inch. When this structure has been exposed to the desired source of radiation, the particles from which are to be counted, the resultant current pulses which flow through the diamond each time a charged particle penetrates it are amplified as shown and the signals thus produced are counted by the device |6 whichinay be adjusted so that only pulses of a given amplitude or greater are inspected for counting purposes. The expedient of the invention, involving the irradiation of the diamond by light, in order to preventor eliminate polarizas tion effects, is taught by the same means as is illustrated in connection with the earlier numbered figures.

Fig. 5 illustrates an amplifier organization of the invention, therefore employing irradiation by light of the solid insulator, which here is a diamond with an evaporated gold electrode on either face as the diamond is presented to the bombarding electrons. This suborganization comprising the diamond and its immediately associated circuits and structures is a part of an organization altogether constituting an amplifier which may be considered as applicants version of the prototype amplifier disclosed in Figs. 6 and 7 of the above Wooldridge patent. This figure, also, is the same as Fig. 6 of the above McKay patent except for the use of the antipolarization means of the invention as compared with McKays alternative means. MoKays Fig. 5 which discloses the vacuum tube and contents of the amplifier of the invention, in specific detail, is omitted from the present application for reasons of simplicity. It should be understood in advance that the signal wave, that is the wave to be amplified, and which "is identified by reference number and the legend Mod., may besinusoidal, a square wave, or a wave of any other conformity. More particularly, having in mind apractical expedient, the organization may be an amplifier of pulse waves.

In Fig. 5 the vacuum tube closure member 32 is comparable with the closure member of a conventional vacuum tube. A vacuum tube is here necessary, as it was not in the earlier numbered figures which illustrates apparatus utilizing alpha and beta particles, because here the charged particles are electrons emanating from the usual cathode source as in conventional vacuum tubes. The reference numerals 23, 24 and 3| together indicate an electron gun of a type which is customary in vacuum tubes which generate and utilize ;a conformed cathode .ray or beam. The gun conforms and directs the beam emanating from cathode .24 onto the solid insulator -25, An .aperturerplate might .well be used to further delimit the beam, .as in the earlier numbered figures. The conditions affecting this solid insulator are similar to those as specific to .the earlier disclosed circuits, the impingement of the electrons being reflected in amplified iorm .as the bombardment induced current inherent in the operation. Polarization is prevented .or'eliminated, exactly as in the earlier disclosed circuits, by the similarly disclosed means which are identified by like reference numerals.

The particular .electron gun in question, as specifically disclosed, conforms to conventional practice for vsuch'electron guns as used for a variety of purposes. The cathode 24 may be indirectly heated, as disclosed, or be of a. filamentary type. The electron emanations are urged outwardly and concentrated by anode 3!, beyond which the beam passes to the solid in.- sulator, here-again assumed tobe a diamond crystal as disclosed. Electrode 23, which other"- wise constitutes a conventional element of an eleetron gun is also the grid control element on which the signal waves to be amplified are impressed frommodulator 26. By it a true density modulation of the electrons in the beam is achieved, as distinguished' from the alternative arrangement of Fig. '6 of the prototype Wooldridge patent in which the modulation is achieved by playing a beam across the crystal so as to change the concentration of electrons incident'on the crystal. I

It will be understood that various other electrodes, for fooussing and accelerating electrons, may beassociated withthe anode 31 or'supplement it as taught bythe prior art, althoughthese accessory elements are not absolutely necessary and, in interest of simplicity, areomitted from Fig.5, The necessary electr'io field for-the solid insulator is provided by the direct voltage source 2 8 and the electron bombardment induced conductivity in said insulator is reflected in a correspondingly varying currentin lead 21, this curren va ng with the c po di g y a ing number "of electrons incident on the solid insulator as determined by the signal source it, The current iniead zl may be amplified by element 29 and indicated by oscilloscope or the like 39, similarly as in the earlier numbered figures. Of course other types of translat ng device than the oscilloscope disclosed may be used.

It should be understood that the amplifying action here concerned has no relationship to the amplifying action inherent in the conventional vacuum tube which the organization would somewhat resernble without the diamond orystal solid insulator. jQn the contrary, the amplifying ac tion results irom the inherent ability of the solid dielectric (diamond e-rystal) under the conditions imposed to become conductive "when bombarded by the electrons. The impressed signal wavespa'rtaire of this amplification, the eventual conductivity current refiecting, in amplified form, such signal waves, Perhaps this phenomenon would be utilized in the form of alight modulator, wherein the variations "in the intensity of the light incidentonthe diamond result in bombardinen-t conductivity which -is-modulated by the light intensity modulations. 1 s

While only a limited number of possible applications of the invention have been disclosed, it

should again be emphasized that the scope of utility of the invention is essentially as wide as that of the utility of the bombardment induced conductivity principle in general.

feet were made. The tungsten filament in a standard 21 C. P. (candle power) lamp was focussed onto the edge of a diamond as in Fig. 2 By means of limiting apertures, the intensity of the illumination could be varied. At approximately .001 C. P. the pulse rate under alpha bombardment was about 7000 per minute and rose rapidly with increasing intensity of illumination. At about .06 C. P. the pulse rate was approximately 23,000 per minute and further increase in input intensity gave no further increase in pulse rate, i. e. light saturation had been achieved.

What is claimed is:

l. The method applicable to a solid insulator which comprises, establishing an electrical field in the region of said insulator, impressing radiation exclusive of low energy electromagnetic waves on said insulator while affected. by said field, while at the same time continuously irradiating said insulator with light, and utilizing the resultant induced electrical conductivity current which, at least in part, flows in said insulator.

2. The method applicable to a solid insulator which comprises, applying an electric field in the region of said insulator, bombarding said insulator with electrically charged particles while afiected by said field, while at the same time continuously irradiating said insulator with light, and utilizing the resultant induced electrical conductivity current which, at least in part, flows in said insulator.

, 3. In the method applicable to a solid insulator which comprises, impressing radiation exclusive of low energy electromagnetic waves on said solid insulator while it is immersed in an electric field, the further steps, in the interest of antipolarization, of irradiating said solid insulator with light and utilizing the resultant induced conductivity current which, at least in part, flows in said insulator.

4. The method applicable to a diamond which comprises, impressing an electric field on said diamond from a Voltage source connected to conducting electrodes or coatings on boundary surfaces of said diamond, impressing radiation exclusive of low energy electromagnetic waves on said diamond, irrespective of direction, while irradiating said diamond with light, also irrespective of direction, and utilizing the'resultant induced conductivity current which traverses at least a portion of said diamond, in the circuit including said source.

The following quantitative measurements on this light ef- 5. The method recited in claim 4 in which the radiation comprises electrically negative particles.

6. The method recited in claim 4 in which radiation comprises alpha particles.

7. The method recited in claim 4 in which the radiation comprises beta particles.

8. In combination, an electrical insulator, means for applying an electric field across at least a portion of said insulator, means for impressing radiation exclusive of low energy electromagnetic waves on said insulator, means for irradiating with light at least a portion of said insulator affected by said field, and a current responsive means in circuit with said field impressing means for indicating the resultant bombardment induced current in said insulator.

9. The combination recited in claim 8 in which said field impressing means is adapted to impress a direct voltage field.

10. In combination, an electrical insulator, conductive coatings on relative opposed surfaces thereof, means for impressing an electric field across at least a portion of said insulator, using said coatings, means for bombarding said insulator with electrically charged particles, and a current responsive means in circuit with said field impressing means for indicating the resultant bombardment induced current in said insulator.

11. In combination, an electrically insulating crystal, two conducting electrodes applied to separated portions of the surface of said crystal, a circuit connecting said electrodes and including a source of voltage for impressing an electric field across the crystal and including a current responsive means, means for bombarding said crystal with electrically charged particles, and

means for irradiating said crystal with light during said bombardment, said current responsive means being adapted to respond to current impulses corresponding individually to the incident corresponding charged particles.

ARTHUR J. AHEARN.

REFERENCES CITED The following references are of record in the file of this patent:

UNTTED STATES PATENTS Number Name Date 2,508,098 Chilowsky May 16, 1950 2,537,388 Wooldridge Jan. 9, 1951 2,543,039 McKay Feb. 27, 1951 OTHER REFERENCES Van Heerden, The Crystal Counter, 1945, pages 11-28. 

